Random storage



April 1964 H. w. FULLER ETAL 3,131,078

RANDOM STORAGE Original Filed May 21, 1958 4 Sheets-Sheet 1 Is I 7 FIG. 4

INVENTORS HARRISON W. FULLER MURRAY E. HALE.

April 23, 1964 H. w. FULLER ETAL 3,131,078

RANDOM STORAGE 4 Sheets-Sheet 2 Original Filed May 21, 1958 5 G u I IS F M 2 O Q 3/ 3 H EIIAIII I I I H A v 8 A 2 A a 9k 0 2 3 2k 3 IS I FIG. 6

INVENTORS HARRISON W. FULLER MURRAY E. HALE- ATTORNEY April 1964 H. w. FULLER ETAL 3,131,078

RANDOM STORAGE Original Filed May 21, 1958 4 Sheets-Sheet 3 Q l .v/

FlG. l2

INVENTORS HARRISON W. FULLER MURRAY E. HALE WEI/" 111,1121, OMMJ ATTORNEY April 28, 1964 H. w. FULLER ETAL 3,131,078

RANDOM STORAGE Original Filed May 21, 1958 4 Sheets-Sheet 4 l/l/l/jj/l/I/l/l/l//////1 //////////////lllllllllllllllllz FIG. l5

INVENTORS HARRISON W. FULLER MURRAY E. HALE A TTORNEY United States Patent 3,131,tl78 RANDGM STQQAGE Harrison W. Fuller, Needham Heights, Mass, and Murray E. Hale, Atkinson, Ndl assignors to Laboratory for Electronics, Inn, Boston, Mass, a corporation of Delaware Original application May 21, 1958, Ser. No. 736,888. Divided and this application June 7, 1962, Ser. No. 2%,871

Claims. (Cl. 117--8) The present invention relates in general to new and improved techniques for processing data to obtain high density data storage and means for implementing these methods, and represents an extension of the principles underlying the invention described in a co-pending application to Harrison W. Fuller, Serial No. 697,058, filed November 18, 1957.

This application is a divisional application of application Serial No. 736,888, filed May 21, 1958, and now abandoned, entitled Random Storage, and assigned to the same assignee as this application.

Present day large capacity data storage systems generally employ magnetic storage devices in the form of tapes, drums and disks, data usually being stored on the surfaces thereof. The data is recorded sequentially so that scanning time is involved in any data retrieval. Scanning is normally accomplished by mechanical motion. In the most advanced present day systems, a data storage density of 1000 to 1500 binary digits per inch is possible, minimum access time being of the order of 300 milliseconds.

It requires, however, the utilization of radically different techniques than those employed in conventional apparatus in order to obtain a storage density materially in excess of that which is presently possible. Certain general requirements must be met in order to accomplish this: The scanning probe, which in the case of present day magnetic recording devices consists of the magnetic head gap, must be small in space extent. The relative mechanical motion of the storage medium and of the probe should be eliminated. A minute probe having little or no dispersion must be propagated and must be capable of scanning a large number of data cells in order to minimize selection costs. Additionally, the volume of apparatus required per unit of stored data must be small. The invention which forms the subject matter of this application fulfills the above stated requirements by utilizing techniques wherein the magnetic field associated with a wall separating oppositely oriented domains of a magnetic medium is employed as a probe for scanning a storage medium.

Accordingly, it is a primary object of this invention to provide new and improved data processing techniques.

It is another object of this invention to provide techniques for processing data wherein the magnetic field associated with one or more inter-domain walls is utilized to divide a data storage medium selectively into an ordered arrangement of magnetized areas representative of the data.

It is a further object of this invention to provide techniques for scanning a data storage medium with the magnetic field of an inter-domain wall which traverses a closely associated scanning medium.

It is still another object of this invention to obtain a storage device having a high data storage density per unit volume of storage medium.

In applying the techniques which form the subject matter of the present invention, a scanning medium and a data storage medium are positioned in close association with each other. The scanning medium has a predetermined easy direction of magnetization and follows a 3,131,078 Patented Apr. 28, 1964 substantially square hysteresis characteristic in response to a magnetic field applied in this direction. In accordance with the most advanced available theories, the electron spin axes of the scanning medium are substantially parallel to an axis of preferred alignment in such a situation. The tendency of the electron spin axes is to remain aligned, i.e. to remain parallel to the preferred axis. This is true even though the spin axis orientation, i.e. the relative north and south poles of respective electron spin axes may be opposed. Each distinct domain of the scanning medium consists of an area of likeoriented electron spin axes and is separated from an adjacent, oppositely oriented domain by an inter-domain wall which constitutes the transition between oppositely oriented domains. In the present invention, these interdomain Walls are established successively in the scanning medium and are made to traverse a given dimension thereof. The concentrated magnetic field associated with one or more of the traversing inter-domain walls, hereafter called the scanning field, thus scans the storage medium which is positioned within the effective range of this field. The effectiveness of such a scan may be variously controlled by modulating the scanning velocity, by selectively opposing the scanning field or by a combination of both, to produce static magnetized areas in the storage medium in accordance with the data to be stored.

Data readout proceeds as follows: The storage medium is scanned with the magnetic field associated with the aforesaid traversing inter-domain wall of the scanning medium. The existence of static magnetized areas in the storage medium slows down the traversing wall and, hence, these areas are detected by sensing wall velocity changes.

These techniques make possible a data storage density greatly in excess of that heretofore possible, one theoretical upper limit being imposed by the thickness of the inter-domain walls. Additionally, the range of the available access time is very large and lends flexibility to the possible applications of these techniques in the solution of different problems.

These and other novel features of the invention together with further objects and advantages thereof will become more apparent from the following detailed specification with reference to the accompanying drawings, in which:

FIG. 1 shows one form of apparatus which uses controlled Wall motion and which demonstrates the underlying principles of the invention;

FIG. 2 illustrates means for applying a switching field to the apparatus of FIG. 1;

FIG. 3 illustrates means for applying an inhibiting field to the apparatus of FIG. 2;

FIG. 4 illustrates preferred means for applying a switching field to the apparatus of FIG. 1;

FIG. 5 is a detail view of FIG. 4;

FIG. 6 illustrates in schematic form the component switching fields applied to the apparatus of FIG. 4;

FIG. 7 illustrates apparatus for depositing the thin films used to form the data tracks which comprise the invention;

FIG. 8 illustrates apparatus for depositing films of graded thickness with the apparatus of FIG. 7;

FIG. 9 illustrates apparatus for applying a perpendicular orienting field during the thin film deposition process;

FIG. 10 illustrates an embodiment of the invention which employs random wall motion;

FIG. 11 shows the scanning medium of FIG. 10 during four successive stages;

FIG. 12 shows the storage medium of FIG. 10 after data has been recorded;

FIG. 13 illustrates the double wall method of scanning;

FIG. 14 illustrates another form of apparatus wherein the easy direction of magnetization of both media is the same; and

FIG. 15 illustrates an embodiment of appparatus which utilizes a multi-domain storage medium.

The description and the use of inter-domain walls is probably simplest to explain in the case of a thin film of ferromagnetic material which may be between 100 A. and 10,000 A. in thickness. Thin films of ferromagnetic material, typically Fe, Ni, Co, MnBi or alloys thereof which have been correctly treated to obtain an easy direc tion of magnetization, i.e. to align the electron spin axes substantially parallel to an axis of preferred alignment, are capable of being magnetized to saturation along the aforesaid easy direction to form a single magnetic domain. Theoretically, in such a domain all the elementary magnets of the film, i.e. all the electron spin axes, are oriented substantially alike. If the magnetizing field is reversed and the field is sufliciently strong, it is possible to reverse the orientation of the entire domain rapidly by domain rotation. In such a case, the elementary magnets reverse their direction by 180 substantially simultaneously. This property of rapid switching, which is of the order of seconds, together with the square B-H characteristic of oriented thin films, is of particular importance in the application of these films to magnetic cores and core matrices, both well known in the prior art. See Journal of Applied Physics 26, p. 975, 1955.

If the reversing field is of an intermediate strength, i.e. somewhat larger than the coercive force H which is required to reverse the magnetization, the domain reversal proceeds more slowly by the mechanism of wall motion. In the latter process, reversal of the elementary electron spin moments of the film begins at one or more nuclei and spreads over the area of the film. During this process the film consists of domains which are saturated to magnetization in opposite directions, the domain boundaries comprising the aforementioned inter-domain walls or Bloch walls as they are commonly referred to. Provided the aligned electron spin axes were initially in the plane of the film, they are caused to rotate out of this plane in the region of the inter-domain wall field. Thus, centrally of the finite wall thickness the spin axes are perpendicular to the plane of the film, while on either side they conform to the mutually opposite orientation of respective bordering domains. Accordingly, intermediate the two sides of the wall the electron spin axes assume every transitional position required to execute a 180 reversal. In certain typical ferromagnetic materials of the kind referred to above, this transition occurs over a distance of about 100 lattice cells which is typically of the order of 1000 A. Inter-domain walls can be formed at one edge of the thin film and can be made to progress across the film in a controlled fashion. They have been observed in a number of ways including tthe method of Bitter (H. J. Williams and R. C. Sherwood, Magnetic Domain Patterns on Thin Films, J our. Appl. Physics 28, 548, 1957), in which a colloidal suspension of magnetic particles on the surface of the film results in the agglomeration of these particles in the strong normal field associated with the wall. Another method has employed the well known magneto-optic effect, as described in Phys. Rev., vol. 100, p. 746, 1955.

The application of moving inter-domain walls as a scanning probe depends on the ability to form and propagate a single wall or a spatially restricted concentration of walls in a controlled way across a film of material. The Bloch wall qualifies for this application by nature of its small spatial extension in the direction of motion, by the absence of dispersion in the course of its motion, by its controllable velocity of motion, and by its high volume efficiency since no magnetic head is required. Other practical advantages include the ability to use the concentrated external magnetic field of the Wall itself for both reading and writing operations, the static, non-volatile, non-des- 4 tructively readable and erasable potentialities and finally the low cost of preparation.

With reference now to the drawings and particularly FIG. 1 thereof, one form of apparatus is shown which illustrates the underlying principles of the present invention. A thin-film scanning medium 11 is separated from a thin-film storage medium 12 by a film of insulation 13. A substrate, e.g. glass, which forms a rigid base for the media is omitted from this drawing for the sake of clarity. If atomic diffusion can be avoided when there is direct contact between the two media the insulation film may be omitted. When the latter is used, it consists of a nonmagnetic, non-conductive material whose thickness is small enough to permit the positioning of the storage medium within the effective range of the magnetic scanning field which is associated with the traversing inter domain walls of the scanning medium. Accordingly, the dimensions shown in FIG. 1 are not intended to be representative of the true dimensions, the thickness of the insulation film being approximately of the same order as that of the media. A material which may be used for the insulation film is SiO, since it can be applied in a special manner discussed hereinbelow. The media consist typically of one of the aforementioned metals Fe, Ni, Co, MnBi or alloys of these, such that the switching or reversal time of the storage medium, i.e. the time required to reorient one electron spin axis by as described above, is large compared to that of the scanning medium. The material of the storage medium permits it to support many long, narrow, closely spaced, static domains and hence its coercive force H is relatively high. Predetermined non-uniformities as well as impurities may also be provided in the storage medium in order to retain the aforesaid static domains. The easy direction of magnetization of the scanning medium is indicated by electron spin axes 14 which are shown to be substantially parallel to a preferred axis of alignment 15, the latter being illustrated in the vector diagram which forms part of FIG. 1. An inter-domain wall 16 divides the scanning medium into two separate domains 17 and 21, having substantially oppositely oriented electron spin axes. The electron spin axis orientation of the transitional area which comprises the aforesaid inter-domain wall 16, is seen to be substantially at right angles to the plane of scanning medium 11, such that the field vector H of the magnetic scanning field is parallel to axis 22. A switching field H which has a time-varying magnitude is applied in the direction shown and may have a uniform field gradient along a given dimension of the scanning medium. In the embodiment of FIG. 1 this dimension is defined by edge 23. As a result, an inter-domain wall is established at edge 24 which traverses the storage medium along the given dimension in the direction of vector 25 while the associated scanning field scans a corresponding dimension of the storage medium.

In lieu of applying a gradient switching field, a timevarying field which is uniform along the dimension defined by edge .13 may be applied while scanning medium 11 has a uniformly tapering thickness along this dimension. If the thinnest portion of the scanning medium is at edge 24, the effect of the switching field is strongest at this edge and the inter-domain wall is established there. The electron spin axes of the storage medium shown in the embodiment of FIG. 1 are substantially parallel to an axis of preferred alignment 22 which indicates the easy direction of magnetization. Since the scanning field H is parallel to this axis, it is capable of reorienting the electron spin axes of a narrow region of the storage medium in accordance with its own vector direction. In the case illustrated in FIG. 1, the latter direction is at right angles. to the plane of the storage medium and upward.

The method of storing data in the apparatus of FIG. 1 consists essentially of dividing the storage medium into static domains separated by static inter-domain walls, where each domain is representative of a portion of the data to be stored. This is carried out by selectively applying the H field to establish an inter-domain wall at edge 24 of the scanning medium, and causing the wall to traverse the aforesaid given dimension of the medium. The effect of the scanning field is to reverse the orientation of the electron spin axes of the storage medium, i.e. to bring about progressively the uniform perpendicular magnetization of the latter as the scann ng field scans at corresponding dimension of the storage medium. Data is written by applying an inlfibiting field H, which opposes the scanning field such that the resultant field H H is too small to exceed the coercive force of the square loop storage medium. The application of H signals occurs in a time sequence in accordance with the data to be stored, each such application causing an area of the storage medium to retain its orientation. Each of these areas depends in extent upon the distance the inter-domain wall traveled during the time interval when the inhibiting field was applied. Thus, if it be assumed that the initial electron spin axis orientation throughout the entire storage medium was as shown in positions 1 through i, i.e. opposite to the direction of the H field, it will be seen that the action of H to reorient these spin axes Was inhibited in positions I) and d, while reorientation took place in positions a, c and e. Thus, the scanning action to the extent illustrated in FIG. 1, produced five static oppositely oriented domains separated by static inter-domain walls, while the positions defined by letters f through 1', as yet unscanned and all oriented in the same direction, still constitute a single domain. It will be evident that data significance may be assigned to each domain so created. For example, the orientation of each domain may be representative of a binary One or a binary Zero. In this connection it should be noted that, where the effect of the scan is the same in two or more successive positions, the resultant area actually forms a single domain. By means of a self-clocked readout, e.g. of the hind described in a copending application to Harrison W. Fuller et al., Serial No. 505,894, now Patent No. 2,972,735, filed May 4 ,1955, it is possible to determine the existence of adjacent like-oriented areas.

The energy obtained upon data readout from a single domain may be too small to produce a usable signal-tonoise ratio. A sequence of oppositely oriented domains may be used for each bit cell, where the particular order of the domains is representative of the binary digit. It should be noted that self-clocked readout is also applicable here.

The latter readout method is dependent upon spatially stabilized bit cells. To provide this condition the double pulse RZ (return to zero) method of recording may be used, as described in the above cited copending application. For example, an unreversed region may precede a reversed region of magnetization to represent a binary Zero, while following the reversed region for a binary One. Such sequences are again recorded by the selective use of the inhibiting field.

In general, once the entire scanning medium assumes the spin axis orientation of domain 17, a new wall can be produced only by reorienting the spin axes in the direction presently assumed by domain 23. In order to bring this about the direction of the switching field H must be reversed. The occurrence of an erasing scan always calls for a scanning field which is reversed in direction from that of the previously occurring recording scan. Similarly, every recording scan must have a scanning field of opposite direction to that of the last occurring erasing scan. Since the direction of spin axis rotation determines the direction of the E vector of the traversing inter-domain wall, it is important under these conditions that successive spin axis rotations of the scanning medium occur in the same direction. Thus, unless the clockwise rotation of the electron spin axes, indicated as occurring between domains 17 and 21, is continued, the next-occurring inter-domain wall, which will reorient domain 17 to the spin axis orientation of domain 21, will have an H vector in the same direction as the one shown in connection with wall 16 of FIG. 1. Such a wall could not erase, i.e. reorient the electron spin axes of the storage medium to bring about the initial orientation as shown at positions f through j. For this reason a small quadrature field which is parallel to axis 22 is employed to assure successive spin axis rotations in the same direction. In the case of FIG. 1, the quadrature field is applied at edge 24, in opposition to H Partial erasing, i.e. the uniform perpendicular magnetization of only a portion of the storage medium may be achieved by applying an erasing scan and opposing its scanning field during a portion of its travel by the application of an inhibiting field.

The data readout operation is carried out by scanning the storage medium while an oppositely directed inhibiting field is maintained in order to prevent any erasing from taking place. The resultant field which then scans the storage medium produces voltage variations in an appropriate sensing winding upon being velocity modulated by the magnetic fields associated with the static interdomain walls of the storage medium. These voltage variations are detected and are indicative of the presence of respective domains and, hence, of the stored data. In an alternative readout method, no inhibiting field is used and the applied switching field causes the wall to propagate at a rate too large to afiect the magnetized areas. As above, the irregular velocity pattern of the traversing wall, which is due to the existence of local fields, is observed by the voltage variations in the sensing winding.

The above methods of detecting oppositely oriented domains can be compared to the well known Barkhausen effect which occurs naturally in ferromagnetic materials. The imperfections which cause velocity modulation of the moving inter-domain walls in this case are represented above by the fields of the data-bearing domains of the storage medium.

FIG. 2 illustrates apparatus for implementing the operation discussed above in connection with the sandwichtype apparatus shown in FIG. 1, applicable reference numerals having been carried forward. A substrate 27, which may be glass, is located under storage medium 12 and lends support to the entire structure. A source V applies a slowly increasing voltage to send an increasing current I through field coil 31 which is wound about the sandwich. Due to the graded thickness of the scanning medium, the effect of the resultant switching field H is to produce an inter-domain wall at the thinnest portion of the scanning medium. The increasing switching field then moves the wall by overcoming the coercive force of the progressively thicker sections of the scanning medium. Terminals 33 are conveniently located to measure a voltage e during readout, which is indicative of the presence of distinct domains, as described above.

FIG. 3 illustrates apparatus for applying the inhibiting field H A current I, is applied to a field coil 34 which is wound around a core 35. The latter has an air gap 36 large enough to contain the sandwich. The field H which exists in the air gap is applied to the entire scanning medium and is at right angles to the plane of the sandwich. The direction of current 1, determines the proper field direction in opposition to H Apparatus for obtaining a switching field gradient is illustrated in FIGS. 4, 5 and 6, applicable reference numerals again being retained. Substrate 27 supports two conductive, non-magnetic films 2% and 29 which may consist of silver and which are separated from each other by a film of insulation. Another insulation film separates the ferromagnetic film of storage medium 12 from conductive film 28. The latter in turn is succeeded by insulation film 13 and by the ferromagnetic film of scanning medium 11. As may best be seen from FIG. 5, a current I is applied from a conductor to a thick block 32 which serves the function of presenting a uniform distribution of current to conductive film 29. The current flow creates a magnetic field H which surrounds the plane of film 29. The field is uniform close to the film surface and is normal to the direction of current flow. As will be apparent from FIG. 6, the H field is also uniform along the path of current flow in the film. Since the scanning medium is positioned within the efiective range of this field, a uniform H field is applied along the dimension of wall traversal. In a preferred embodiment film 29 overlaps every edge of the sandwich in order to prevent end efiects due to its magnetic field. Where the latter are sufiiciently small, all the films of the embodiment of FIG. 4 may cover the same area. A current 1 is applied to block 3t? which presents a uniform Current distribution to film 28. The latter is tapered in width and similarly overlaps every edge of the sandwich. The field H, which is due to the I current, differs from the H field by being of opposite direction in the vicinity of the scanning medium and having a field gradient along the dimension of wall traversal. The resultant switching field H which is applied to the scanning medium, creates a region 31 intermediate strong positive and negative fields where the magnetic field strength is less than the coercive force of the medium. It has been determined that such a region requires the existence of an inter-domain wall and hence, passing region 31 across the scanning medium will determine the origin as well as the traversal of the wall. By increasing the magnitude of I in time, the resulting H will vary such that the aforesaid region traverses the scanning medium and thereby controls the motion of the inter-domain wall. It will be evident that numerous structural variations are possible for obtaining the same result. For example, positioning the sandwich intermediate two conductive films, which are connected together along one edge, permits the use of a single current source. Still further refinements are possible by using a third conductive film.

Regardless of the number of conductive films used for each sandwich, or their position relative to the sandwich, successive sandwiches with their associated conductive and insulation films can be built up by using a single substrate as a base, current connections to respective conductive films being made along the edges.

In the case of the embodiment illustrated in FIG. 4, a vacuum evaporation process may be used to deposit every one of the films. FIGS. 7, 8 and 9 illustrate apparatus for carrying out the vacuum deposition process. A bell jar 61 positioned on base 62 is adapted to be evacuated through a hole 63 to form the vacuum chamber. Four pairs of posts 64, 65, 6d and 67 are fixed in the base, the posts of each pair being bridged by a tungsten filament. Electrical connections are provided on the underside of base 62 in order to pass current through the filaments for heating the latter. Each of the tungsten filaments holds a different material. Thus, filament 71 may hold a conductive, non-magnetic material, e.g. silver; filament 72 may hold a high coercive force material, e.g. manganese bismuth; filament 73 may hold an insulating material, e.g. magnesium fluoride or sfiicon monoxide; and filament 74 may hold a low coercive force material, e.g. Permalloy. The various materials recited are held in their respective filaments by capillary attraction and by surface tension when in a molten state. A masking disk 75 is positioned on shaft 76, the latter being rotatably arranged in base 62. The disk contains a number of apertures 77, the shape of each aperture being determined by the particular area to be coated. A substrate 81 is held in position above the masking disk such that apertures 77 may rotate past the substrate. A heating element 82, adapted to receive currents from posts 83, is disposed adjacent the substrate in order to heat the latter. A pair of field coils $4 and 85 are positioned outside the vacuum chamber and are adapted to set up a magnetic orienting field in a direction perpendicular to shaft 7 6.

FIG. 8 illustrates an arrangement for applying an orienting field in a direction parallel to shaft 76, applicable reference numerals hav ng been carried forward. As shown in the drawing, a field magnet 39 is positioned above the masking disk with its core axis aligned parallel to the supporting shaft of the latter. A film 85, of graded thickness, is shown deposited on substrate 81.

FIG. 9 illustrates apparatus for obtaining a coating of graded thickness. A uniformly rotating masking disk 75 is employed which has one or more apertures 77 cut in the shape of a triangle. As the aperture rotates past substrate 81, upper edge as of the latter will be exposed to the coating material for a sh rter time period than lower edge 87, due to the narrowing dimension toward the apex of the triangle. Edge 87 will be exposed the maximum amount of time permitted by the aperture. Intermediate these two edges a coating of a graded thickness will be deposited on the substrate.

In order to deposit the various coatings shown in the embodiment of FIG. 4, a suitable aperture of the masking disk is rotated under substrate 31 in FIG. 7 to coat the precise area desired. The substrate is maintained at a suitable temperature by heating element 82 so that evaporating materials may be readily deposited thereon. Current is applied to posts 64, whence the silver on filament 71 is melted and is evaporated. Only that area of the substrate which is exposed by the masking aperture is coated by the evaporating silver which readily adheres to the heated surface. Upon completion or" the deposition of the silver coating on the bare surface of the substrate, an aperture exposing an area smaller than that of the last aperture is rotated under the now coated substrate. As previously explained films of successively smaller area are deposited only when magnetic field end effects must be considered. A current is applied to posts 66 and the silicon monoxide on filament 73 is evaporated and is deposited on the silver coating previously applied to the substrate. When the insulating coating has been built up to the desired thickness, the process is terminated. Re spective second coatings or" silver and silicon monoxide are successively deposited, each coating being masked to cover a successively smaller area. Following the deposition of the last coating of insulation the proper aperture for the storage medium is rotated into position. Alternatively, a medium of equal area as the last applied film is deposited by using the same masking aperture. An orienting field normal to the substrate is applied by means of field magnet 89. Current is applied to posts 65 in order to evaporate the manganese bismuth in filament 72. As a result of the applied normal orienting field, the deposited film of manganese bismuth will have a uniaxial anisotropf, tie. it will possess an easy direction of magnetization in the direction of the applied field. After a uniform film of the storage medium has been deposited a third coating of insulation is deposited in the manner described hereinbefore. Subsequently, the application of current to posts 67 causes the Permalloy on flament 74 to evaporate and to be deposited through the desired masking disk aperture on top of the last applied coating of insulation. If it is desired to deposit a scanning medium having a graded thickness, an additional disk of the type illustrated in FIG. 9, must be rotated at a uniform rate between the maslc'ng disk and the sample to be coated during this portion of the deposition process. Simultaneously with the deposition of the scanning medium, field coils 84 and 85 apply an orienting field substantially in the plane of the substrate to produce a film having uniaxial anisotropy as before, the easy axis of magnetization being aligned with the direction of the applied field. In view of the fact that the storage medium previously deposited uses a high coercive force magnetic material, it is not affected by the subsequent application of an orienting field in the plane of the substrate.

If it is desired to give the storage medium an easy direction of magnetization parallel to the plane of the substrate, orienting field coils 84 and 85 are used during the deposition process of the storage medium. Depending on whether the easy axis of magnetization is to be parallel or transverse to that of the scanning medium, the field coils may be suitably positioned relative to the sample being coated. In the event that a low-anisotropy storage medium is desired, as explained hereinbelow, no orienting field is applied during the deposition of the latter.

The deposition process described above may also be used to deposit blocks 3% and 32 on their respective conductive coatings. Special masking must be used to carry out this process and a substantially longer period of deposition is required. It will be understood that a stack of data tracks of high volume efiiciency may be built up by successively depositing films of the proper materials on top of each other in the order described above. Each data track thus deposited comprises a storage medium film, a scanning medium film, and the corresponding conductive and insulating films. Care must be exercised to apply the appropriate magnetic fields during the deposition of each medium.

Except in the case where the scanning medium is to be graded, all films are applied with a uniform thickness. Depending on the requirements of each case, the latter may vary from 180 A. to 10,000 A. Blocks 3t? and 32 are conductively afixed to their respective conductive films either by the deposition process or after the latter is completed. Conductive leads are then attached to the blocks. The area of the sandwich will depend on the particular situation, but may be limited by the dimension which the inter-domain wall can traverse reliably. Thus, an excessive tendency of the wall to curve during its traversal of the scanning medium may be undesirable when it impairs the reliability of the scan. Since such curvature is sometimes aggravated by an excessively long traversal, there may be a limitation on the area of the scanning medium. It should be noted, however, that wall curvature is not damaging as such, provided it is reproducible on successive scans.

One method of inhibiting the curvature of the interdornain walls is to provide straight line discontinuities in the storage medium normal to the direction of travel of the scanning field. These may, example, take the form of grooves at intervals equivalent to the expected storage density, wh ch may be produced during the vacuum deposition process. Alternatively, deposition of the storage medium can be performed on a d ffraction grating replica. Final polishing of the high spots produces true discontinuities, and the uniform insulation as well as the scanning medium are deposited thereon. The advanced sections of the curved traversing wall will then be delayed sufiiciently at each discontinuity to enable the lagging section to catch up. Since the average Wall velocity may be properly controlled, these regularly spaced discontinuities produce periodic variations in the applied switching voltage. These variations may be sensed during the data writing process to furnish an external clock.

In order to obta n reliable data readout, it is important that the storage medium retain its static inter-domain walls in the same position where they were formed during the data storage process. The use of a high coercive force material for the storage medium may not alone be sufiiciently reliable to accomplish this in the presence of external fields. To this end, finely divided impurities may be deposited together with the material of the storage medium, which will aid in trapping the Walls to keep them stationary. Alternatively, the storage medium may be applied through a finely divided screen which creates minute discontinuities between metallized spots.

It will be remembered, on the other hand, that freely movable walls are desired in the scanning medium and hence a low coercive force material is used. Care must be taken to shield the scanning medium from the efiects of external fields, including the earths magnetic field.

In the above described data processing technique, velocity modulation of the traversing inter-domain Wall may be substituted for the use of the inhibiting field in order to control the effectiveness of the scanning field during storage as well as during readout. In this method, the inter-domain wall normally traverses the scanning medium at a velocity too great for the associated magnetic scanning field H to have any effect on the storage medium. Data is written at the desired positions of the storage medium by decreasing the velocity of motion of the intendomain wall. This permits the H field to reorient the aflected area of the storage medium. Taking the embodiment of FIGS. 5 and 6 as an example a binary One is written by opposing the time-varying switching current I such that its rate of increase is temporarily slowed down. As a result, region 31 travels across the scanning medium at a slower velocity and causes the scanning field to linger long enough to reverse the perpendicular magnetization of the storage medium. Thereafter, the original velocity of the wall is resumed. In the case described, binary zeroes are represented by the non-reversal of the perpendicular magnetization of the storage medium. In order for this method to be successful, the scanning medium should have a small reversal time relative to that of the storage medium.

It is a property of thin-filrn media that variations in the uniformity of the film such as thin spots, scratches or impurities can serve as nuclei for the origin of random intendornain walls upon the application of a switching field. These non-uniformities may cause variations in the local direction of uniaxial anisotropy, i.e. variations in the local easy direction of magnetization. For example, local stresses in the scanning medium may cause the local magnetostrictive anisotropy to overcome that induced by the orienting field with the result that the interdomain wall in the affected areas as compared to the remainder of the medium, will not be parallel to the easy direction of magnetization. This property of non-uniform films is taken advantage of in a preferred embodiment of the invention illustrated in FIGS. 10, 11 and 12. In the application of this principle it is not important where in the scanning medium the nucleus of such a random inter-domain wall is located nor, indeed, how many nuclei exist. It is only important that the walls be capable of being faithfully reproduced upon the successive application of a switching field. The domain wall motion, which is a function of the random field gradient which exists, need not proceed from a given point nucleus in a generally outward direction, but may also occur in a manner where the area surrounded by the domain wall decreases. Indeed, it is possible that within the area defined by a decreasing domain wall a nucleus exists which gives rise to a domain wall that moves in a direction generally outward from the last recited nucleus so as to meet the domain wall of the aforesaid decreasing area. In such a case the domain walls disappear to form a single domain. Similarly, two outwardly moving domain walls may meet and disappear. As previously pointed out, in order to utilize the phenomenon of random Wall motion upon the application of a magnetic switching field, it is important that these walls be made to occur in a reprodncible manner. In the embodiment illustrated in FIG. 10, the area of scanning medium 11 exclusive of non-uniformities has an easy direction of magnetization substantially in the plane of the medium and further contains an inter-domain wall 16 which is assumed to be moving in a generally outwardly direction from a nucleus located somewhere within domain 21. With the exception of scanning film 11 which contains the aforesaid nonuniformities, the construction of this embodiment follows closely that of the sandwich illustrated in FIGS. 1, 2 and 3. The substrate has been omitted from FIG. 10 for the sake of clarity. The drawing illustrates the case where the original spin axis orientation 61, as indicated by domain 17, has been reversed by outwardly moving wall 16 to the orientation 62 of domain 21. As in the example of FIGS. 1, 2 and 3, the application of inhibiting 11 field H modulates the effect of scanning field I-l upon storage medium 12, the latter having an easy direction of magnetization perpendicular to the plane of the medium.

FIG. 11 illustrates four successive stages in the motion of random inter-domain walls which occur at two general locations A and B of the scanning medium of FIG. 10. The application of switching field H causes a wall to originate at a nucleus located somewhere outside of area 1 at locations A and B respectively, and to shrink in a generally radial direction. It will be seen that area 3 at location B splits into two separate areas during stage 4. The spin axis orientation of the areas encircled by the inter-domain walls, relative to that of the remainder of the scanning medium, is indicated by the direction of the arrows.

FIG. 12 shows the efiect which may be achieved at locations A and B respectively, of storage medium 12 by utilizing the above-described randon inter-domain wall motion in cooperation with an inhibiting field. The selective application of the inhibiting field H during the second and the fourth stage prevents spin axis reversal in the storage medium. Thus, oppositely oriented domains 1, 2, 3 and 4 respectively are produced at each of locations A and B, where domains 2 and 4 retain their original spin axis orientation. If, in the manner explained hereinabove, binary digital significance is assigned to the orientation of distinct bordering domains, it will be seen that alternate binary Ones and Zeros are stored at the two locations of the storage medium.

Another technique which may be employed to process data, is the use of a spatially restricted concentration of walls as the scanning probe. In the example illustrated in FIG. 13 a double scanning wall is employed. If a pair of closely spaced inter-domain walls is established in the scanning medium by rotating the electron spin axes thereof 27r radians, the external fields normal to the scanning medium associated with the respective walls are of opposite direction. This is illustrated at 41 and 42 of FIG. 13. Accordingly, a strong longitudinal field I-I is created in region 43 which may be utilized to magnetize the storage medium. Since this field has a direction which coincides with the direction of motion of the pair of walls, a corresponding easy direction of magnetization of the storage medium is required. Thus, the preferred axis of alignment of the storage medium lies in the plane of the medium and is normal to that of the scanning medium. Accordingly, it is possible to establish distinct adjacent domains in the storage medium whose spin axes are substantially oriented either in the direction of wall motion or 180 reversed therefrom.

Both in the embodiment of FIG. as well as in the embodiment of the invention described in connection with FIG. 13, a certain measure of independence is maintained since the preferred axes of alignment of the two media are normal to each other. Another embodiment of the invention which is applicable to random wall motion is illustrated in FIG. 14 where storage medium 12 is seen to have an axis of preferred alignment which is identical to that of scanning medium II. In this embodiment, the motion of the electron spin axes 14, as they are reoriented by the traversing inter-domain wall 16, is relied on to reorient the electron spin axes of the storage medium. An inhibiting field may be used to control the scan, or alternatively, velocity modulation of the wall motion may be employed. In the latter case, the required reversal time of the scanning medium should be considerably lower than that of the storage medium. FIG. 14 shows positions a through j, data having been stored at positions a through e. A circle with a dot indicates the head of an arrow, i.e. a spin axis orientation equivalent to that of domain 21, while a cross indicates an orientation equivalent to that of domain 17.

The techniques hereinabove discussed are not confined to the processing of digital data. If it is desired to store data corresponding to an analog quantity, the representative analog signal may be readily transformed by the well known expedient of pulse width modulation into a signal capable of being used by the apparatus described above. Thus, using the apparatus illustrated in FIG. 3, the pulse Width modulated signal which is representative of the analog quantity is applied to inhibiting winding 34. Where velocity modulation is employed, as illustrated in FIG. 5, the signal is used to modulate current I In either case, the resultant static domains of the storage medium vary in width, such variation being representative of variations in the magnitude of the analog quantity. Data readout is readily carried out by the velocity modulation scanning method described above, whereby the width of the aforesaid domains is detected.

In an alternative technique of processing analog or digital data, a multi-domain storage medium is substituted for storage medium 12 of FIG. 10, respective domains of said storage medium having diverse electron spin axis orientations. See FIG. 15. Non-square loop material is employed for the storage medium which may or may not have a muiti-crystalline structure. For example, such a storage medium may comprise a low-anisotropy material consisting of micron sized particles of iron oxide applied in the form of a thin film as hereinabove described, or sprayed on a support such as tape. Utilizing the thin-film scanning medium 11, the scan or" field H due to traversing wall 16, when either velocity modulated or selectively inhibited, will establish areas in the storage medium which have varying efifective strengths of magnetization in accordance with the data which is to be stored. It will be noted that respective domains of the storage medium, which are indicated by circles and an orientation vector, are not completely aligned in area 39 which is under the influence of the H field. In the instant case this is immaterial, since it is the efiective strength of magnetization of the area which is representative of the amplitude of the stored signal. As in the case of the above described embodiments, a storage scan must be preceded by an erasing scan. Area 4%, which has not yet been scanned by the writing wall traversing in direction 25, shows the magnetization due to a previously traversing erasing wall. In the case of digital data readout proceeds in the manner described above, while for analog data the variation of field strength encountered by the scan is detected as a varying signal representative of the analog quantity. Thus, by utilizing the techniques described herein, data storage of a digital or an analog quantity is possible at a density far in excess of that possible with conventional apparatus.

The techniques discussed herein with respect to a specific embodiment are generally applicable to the other embodiments illustrated and described. Thus, either wall velocity modulation or an inhibiting field may be used in each of the embodiments of the invention to control the effect of the scan on the data storage medium. Similarly, either the single wall or the double wall method of scanning may be used. Numerous modifications are also possible in the application of the switching fields as well as in the type of media used both for scanning and for storage, all as discussed herein. In general, these modifications are governed by the specific requirements of each situation.

The storage techniques described and illustrated hereinabove, permit a data storage density, both of digital as well as of analog data, far in excess of that possible with present day equipment. These high data storage densities are achieved at no sacrifice in access time. On the contrary, the scanning wall can be made to move with almost arbitrarily great velocity, such velocity being limited by domain rotation for the complete reversal of the film at approximately 10- seconds, rather than by any inherent limitations in the scanning process.

This range of controllable scarming velocities is indicative of the extremely high scanning rate which may be used until the required record is found. But since a much faster scanning rate implies an impossibly high bit rate (from a practical frequency standpoint), the application must employ a much reduced linear bit packing density for the rapidly scanned identifying record tags. When the required record is reached, the scanning rate can be slowed down to take advantage of the high bit densities possible. In this high-capacity, fast access time device, it is necessary that the record tag lengths be dimensionally negligible in spite of their exaggeration, and that the ratio of scanning rates be large enough so that normal information signals can be eliminated by low pass filtering when the file is being searched.

Having thus described the invention, it will be apparent that numerous modifications and departures, as explained above, may now be made by those skilled in the art, all of which fall within the scope contemplated by the invention. Consequently, the invention herein disclosed is to be construed as limited only by the spirit and scope of the appended claims.

What is claimed is:

l. The method of manufacturing data processing apparatus comprising the steps of vacuum-depositing a first thin film of high coercive force ferromagnetic material on a difiraction grating replica supported by a substrate, applying a first magnetic field during the deposition of said first ferromagnetic film to provide the latter with an easy direction of magnetization parallel to a first axis, abrading the high spots deposited to produce true discontinuities in said first ferromagnetic film, vacuum-depositing an insulation film on top of said first ferro-magnetic film, vacuum-depositing a second thin film of low coercive force ferromagnetic material on top or" said insulation film, and applying a second magnetic field during the deposition of said second ferromagnetic film to provide an easy direction of magnetization of said second ferromagnetic film parallel to a second axis.

2. The method of manufacturing data processing apparatus comprising the steps of vacuum-depositing a first thin film of high coercive force magnetic material on a supporting substrate, applying a coercive magnetic field during the deposition of said first film to give the latter an easy direction of magnetization parallel to a predetermined axis, vacuum-depositing a film of insulating material on said first film, vacuum depositing a second thin film of low coercive force magnetic material on said insulating film, and applying a coercive magnetic field dur- Mi ing the deposition of said second thin film in a direction normal to said first-applied field to give said second thin film an easy direction of magnetization normal to said predetermined axis.

3. The method of manufacturing a data track including at least one insulated film of conductive material, comprising the steps of heating a substrate, vacuum depositing a film of non-magnetic conductive material on a planar surface of said substrate, vacuum depositing a film of insulating material on said conductive film, applying a coercive magnetic field in a direction substantially normal to said planar substrate, vacuum depositing a film of high coercive force magnetic material on the last-deposited insulating film during the application of said magnetic field, vacuum depositing a film of said insulating material on said film of high coercive force material, applying a coercive magnetic field substantially in the plane of said substrate surface, vacuum depositing a film of low coercive force magnetic material on said last-deposited film of insulation during the application of said last recited magnetic field, each of said films being deposited through a stationary masking aperture intermediate the source of the material and the surface to be coated.

4. The method recited in claim 3 and further comprising the step of moving at least one tapered aperture past said substrate during the deposition of said low coercive force magnetic film to apply the latter in a graded thickness.

5. The method of claim 3 and further comprising the steps of vacuum-depositing a film of insulation on said film of low-coercive force magnetic material, the films so deposited comprising a first data track, vacuum-depositing the films of successive data tracks on top of said first track, respective high and low coercive force magnetic films of each track being deposited under the infiuence of their corresponding magnetic fields to provide appro priate easy directions of magnetization.

References Cited in the file of this patent UNITED STATES PATENTS 2,853,402 Blois Sept. 23, 1958 2,900,282 Rubens Aug. 18, 1959 2,919,432 Broadbent Dec. 29, 1959 3,015,807 Pohm et al. Jan. 2, 1962 3,019,125 Eggenberger et al. I an. 30, 1962 

1. THE METHOD OF MANUFACTURING DATA PROCESSING APPARATUS COMPRISING THE STEPS OF VACUUM-DEPOSITING A FIRST THIN FILM OF HIGH COERCIVE FORCE FERROMAGNETIC MATERIAL ON A DIFFRACTION GRATING REPLICA SUPPORTED BY A SUBSTRATE, APPLYING A FIRST MAGNETIC FIELD DURING THE DEPOSITION OF SAID FIRST FERROMAGNETIC FILM TO PROVIDE THE LATTER WITH AN EASY DIRECTION OF MAGNETIZATION PARALLEL TO A FIRST AXIS, ABRADING THE HIGH SPOTS DEPOSITED TO PRODUCE TRUE DISCONTINUITIES IN SAID FIRST FERROMAGNETIC FILM, VACUUM-DEPOSITING AN INSULATION FILM ON TOP OF SAID FIRST FERRO-MAGNETIC FILM, VACUUM-DEPOSITING A SECOND THIN FILM OF LOW COERCIVE FORCE FERROMAGNETIC MATERIAL ON TOP OF SAID INSULATION FILM, AND APPLYING A SECOND MAGENTIC FIELD DURING THE DEPOSITION OF SAID SECOND FERROMAGENTIC FILM TO PROVIDE AN EASY DIRECTION OF MAGNETIZATION OF SAID SECOND FERROMAGNETIC FILM PARALLEL TO A SECOND AXIS. 